In paper-making and cellulose machines and machines for similar purposes single-layer metal screens and single or double layer fabrics woven from monofilaments or multifilaments are used in the sheet formation section. Metal screens are not very wear-resistant and are being replaced to an increasingly large extent by wires made from synthetic fibre threads, so called synthetic wires. Although synthetic fabrics are more durable than are metal screens there is a continuous effort to develop them further. Single-layer fabrics woven in three or four shaft patterns exhibit two surfaces that differ in pattern. On one of these surfaces the warp thread binds over two or three weft threads, respectively, whereas the warp thread binds below one weft thread only for each pattern repeat. This surface is usually called the warp side. The opposite surface is characterised in that the weft thread binds over two or three warp threads, respectively, whereas it passes beneath one warp thread only for each pattern repeat. This side is usually called the weft side. When single-layer syntethic wires began to replace the metal screens they were run in the same manner as metal screens, i.e. with the warp side facing inwards towards the abrasive elements of the paper machine. The warp thread, having more crimp was positioned further towards the two external planes of the fabric than was the case with the weft thread. Whichever way the fabric was turned, the warp yarn therefore was the one to be most exposed to wear. Because the warp side contains a larger quantity of warp yarn than does the weft side, it was natural to turn this side inwards towards the abrasive elements. In the case of flat woven and spliced wires, the warp yarn forms the longitudinally extending yarn and takes the entire wire tension whereas the weft yarn forms the crosswise yarn and is practically tensionless during the run in the paper machine. The wearing out of the warp yarn therefore is more limited than in the case of the weft yarn. By stretching the single-layer wire heavily while exposing it to a heat fixation treatment, the crimp may be shifted in such a manner that the warp thread is straightened while the weft thread takes on an increasing undulating configuration or curvature. Following a certain degree of stretching, the weft thread will form the wire surface (primarily on the weft side) whereas the warp thread, which is becoming more and more straight, will be positioned more towards the centre of the wire as seen in the cross-section thereof. When a fabric of this kind is run in the machine, the weft side is turned towards the abrasive elements. Consequently, primarily the weft threads will be exposed to wear. As these threads are practically completely relaxed, the wear may continue for a longer period of time before it becomes necessary to remove the wire from the machine.
In double-layer fabric (wire) structures comprising two layers of weft threads and warp threads interconnecting the layers the situation is different. The geometrical structure of the double-layer fabric hitherto used is such that no crimp or curvature shift corresponding to the one outlined above is possible as a result of stretching. An early type of double-layer forming fabric is characterised in that each warp thread binds in sequence between a first pair of weft threads above a second pair and a third pair, between the threads of a fourth pair and beneath the threads of a fifth pair and a sixth pair before the procedure is repeated. Upon stretching of the warp threads the contact pressure acted on the weft threads in a direction towards the fabric centre. As no counter-acting forces exerted a pressure in a direction outwards towards the fabric surface such stretching operations resulted in the weft threads being forced deeper inwards towards the fabric centre. The conditions are the same in the fabrics of the kind generally denominated paper makers dryer cloths. In this structure each warp thread in succession binds between a first pair of weft threads, above a second pair, between a third pair and beneath a fourth pair before the procedure is repeated.
The performance of the double-layer fabric is to a large extent dependent on the geometrical structure being such that the weft thread, upon stretching of the warp threads, will form the surface layer on both the surface facing the paper (the paper making side) and the one exposed to wear (the wear side). On the paper making side it is desirable that the threads of the two yarn systems will be positioned at essentially the same level, i.e. that their upper peripheral surfaces are tangents to the plane which forms the surface plane of the fabric side facing the paper web. On the wear side of the fabric it is desirable that the outer peripheral surface of the weft thread is positioned in a plane projecting somewhat beyond the plane forming the outer peripheral surface of the warp thread, i.e. that wire is new, its surface in contact with the abrasive elements of the paper machine (suction boxes, foils, and so on) is to be formed by weft threads.
In prior-art fabrics (wires) having this geometrical structure it is known that the layer of weft threads which in position of use is turned towards the material to be formed and the warp threads interconnecting these layers, are essentially tangents to the plane of the fabric (the outer plane) facing said material (Swedish Published Specification No. 366,353). The plane of the wire facing the dewatering elements of the paper machine (the inner plane) was earlier designed in such a manner that each warp thread bound beneath two successive pairs of weft threads. Because of this structural design it becomes impossible to achieve, by stretching the warp threads and partly straighten their crimp or curvature, a sufficiently high degree of further undulation of the layer of weft threads which faces the dewatering elements for these threads to be positioned beyond the plane formed by the peripheral surface of the warp threads. The Swedish Published Specification No. 366,353 does not describe any particular structural design for the part of the fabric facing the dewatering elements, and consequently the present invention must be considered as a further development of the invention disclosed in the publication referred to.